OP: it was a two factor decision.

The first is that it seemed at the time that Scheme was going to be easier to work with, and that has proven true. It's a lot easier to do music theory hacking and integrate with Max using a lisp 1. Being able to seamlessly change whatever the first form is (function, hash-table, list, vector) with no change to the code has turned out to be really really nice in my work.

The second was that when I started I knew little about Schemes and Lisps and it seemed when going through the docs like ECL at that time was going to be a bigger lift to get going.